1. Field of the Invention
The present invention is directed to processes for the synthesis of di(meth)acrylic acid esters by transesterification, which permit the recovery of esters that are free of high levels of undesirable contaminants such as zirconium. Preferred processes involve the transesterification of (meth)acrylic acid esters of C1 to C4 alcohols with 1,n-diols (where nxe2x89xa73) in the presence of chelates of metal compounds as catalysts. Most preferably, chelates of zirconium with 1,3-dicarbonyl compounds are employed as the catalytic metal compounds.
2. Description of the Related Art
Di(meth)acrylic acid esters are generally obtained by the catalyzed transesterification of (meth)acrylic acid esters. Conventional metal catalysts for this transesterification are widely known among those skilled in the art. For instance, alkali metal catalysts such as lithium and calcium hydroxide are used, as described, for example, in German Unexamined Applications DE-OS 3423441 and 3423443. However, the use of these basic catalysts may lead to undesireable side reactions such as the Michael addition, which diminish both the purity and yield of the desired di-esters.
Zirconium complexes may also be used for catalysis of reactions between the esters and alcohols. Since these catalysts are neutral these complexes provide extremely high conversions and high purity of the products. A further advantage of such catalysts is that the alcohols do not have to be dried prior to transesterification. Such catalysts are described in German Unexamined Application DE-OS 2805702 and European Patent EP 0236994 B1. Zirconium catalysts may also be formed in situ. For example, FR A 2747675 describes a process for transesterification of (meth)acrylates in which zirconium catalysts are formed in situ.
Whether formed in situ or merely added to a reaction mixture, it is desireable to remove such catalysts as completely as possible after transesterification to provide a non-turbid product free of highly reactive zirconium catalysts. Di-ester products free of highly reactive zirconium catalysts are more conveniently used in subsequent reactions because they do not risk introducing undesireable effects associated with presence of these highly reactive catalysts. Accordingly, it is desireable to separate zirconium-containing catalysts from the di(meth)acrylic acid esters produced by transesterification prior to sale.
Prior methods for removing these catalysts required complex, expensive and inconvenient separation steps. For instance, separating the catalyst required hydrolysis and centrifugation, or in many cases distillation to obtain non-turbid products which had the purity required for numerous applications.
An object of the present invention is to provide an inexpensive and convenient process of obtaining di(meth)acrylic acid esters in highly pure form. The present invention thus provides processes for synthesis of di(meth)acrylic acid esters by transesterification of (meth)acrylic acid esters of C1 to C4 alcohols with 1,n-diols (where nxe2x89xa73) in the presence of metal compounds as catalysts. Preferably, chelates of zirconium with 1,3-dicarbonyl compounds are used as the metal compound. These processes produce di(meth)acrylic acid esters inexpensively and in highly pure form.
Another object of the invention is to provide a process in which the catalyst can be separated from the transesterification reaction product without energy-intensive distillation. The inventors have found that this may be achieved by precipitating the metal catalyst with phosphoric acid after transesterification. The resulting precipitate can be conveniently separated from the ester-containing reaction mixture without distillation. Accordingly, a process is provided for the synthesis of di(meth)acrylic acid esters by transesterification of (meth)acrylic acid esters of C1 to C4 alcohols with 1,n-diols, where nxe2x89xa73, in the presence of metal compounds as catalysts, wherein chelates of zirconium with 1,3-dicarbonyl compounds are used as the metal compound. This process provides the desired di(meth)acrylic acid esters inexpensively in highly pure form without significant amounts of contaminating zirconium compounds which can interfere with subsequent chemical reactions.
Other advantages of the claimed invention include:
The inventive processes lead to extremely high conversions and high purity of the products.
The 1,n-diols used, where nxe2x89xa73, do not have to be dried before they are used.
Inexpensive zirconium compounds can be used in the process, since the catalyst can be synthesized in situ by addition of 1,3-dicarbonyl compounds.
After the zirconium compounds precipitated by phosphoric acid have been separated, it is no longer necessary to purify the end product by distillation.
Products free of turbidity are obtained by the inventive process.
The notation di(meth)acrylic acid esters includes diesters of methacrylic acid, acrylic acid and mixtures of the two acids.
Di(meth)acrylic acid esters that can be synthesized in the scope of the present invention may be generally represented by the formula: 
where
R1 and R2 can be the same or different and denote a hydrogen or a methyl group,
Y denotes a divalent link group, wherein the two (meth)acrylic acid groups are separated by at least 3 carbon atoms. These link groups are derived from the 1,n-diols used for transesterification.
Examples of particularly preferred link group xe2x80x9cYxe2x80x9d are straight-chain, branched or cyclic alkyl groups, which can be saturated or unsaturated, such as propyl, butyl, pentyl, hexyl, cyclopentyl, cyclohexyl, pentenyl as well as polyether glycols. Link groups can contain reactive groups, examples of which include halogen-containing groups, epoxy groups, aromatic and heteroaromatic groups as well as thiol groups.
Usable (meth)acrylic acid esters of C1 to C4 alcohols within the scope of the invention may be represented by the formula 
where
R1 is hydrogen or a methyl group and R3 is an alkyl group with 1 to 4 carbon atoms.
Examples of alkyl groups with 1 to 4 carbon atoms are methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl and tert-butyl. These groups can be unsubstituted or substituted. Either acrylic acid esters or methacrylic acid esters, or mixtures of both can be used in the transesterification.
Commercially available (meth)acrylic acid esters can be used in the inventive process with methyl acrylate and methyl methacrylate being preferred, as these substances are particularly inexpensive. Furthermore, the methanol liberated during the transesterification of these compounds can be easily removed from the reaction mixture by distillation, allowing very high conversions to be achieved.
1,n-Diols (where nxe2x89xa73), include in particular compounds of the formula:
HOxe2x80x94Yxe2x80x94OHxe2x80x83xe2x80x83(III)
where Y has the same meaning as in formula I.
The preferred 1,n-diols include among others alkoxyalkanediols, alkenoxyalkanediols, alkenediols, glycols, polyether glycols, phenoxyalkanediols, alkylphenoxyalkanediols, phenylalkanediols, alkylphenylalkanediols, alkylmorpholinoalkanediols, alkylpiperidinoalkanediols, pyridylalkanediols, and haloalkanediols.
Preferred 1,n-diols include 1,3-propanediol, n-butane-1,3-diol, 2-methyl-1,3-propanediol, neopentyl glycol (2,2-dimethyl-1,3-propanediol), 1,4-butanediol, triethylene glycol and polyethylene glycol 400. Most preferrably the 1,n-diol is characterized in that n=3,4or 6.
The 1,n-diols can be used alone or in the form of mixtures. In general they are commercially available, and their synthesis is widely known among those skilled in the art.
The chelate complex compounds of zirconium with 1,3-dicarbonyl compounds used as catalysts in the transesterification are well known to the person skilled in the art.
The 1,3-dicarbonyl compounds that can be used with particular success in the scope of the invention include among others acetoacetic esters, acetylacetonate, 2,4-hexanedionate, 3,5-heptanedionate, 3-methylacetylacetonate, 3-phenylacetylacetonate, 4,4,4-trifluoro-1-phenyl- 1,3-butanedionate, 2,2,6,6-tetramethyl-3,5-heptanedionate, 1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedionate, 1,1,1 -trifluoro-2,4-pentanedionate and dibenzoylmethane. Acetylacetone is especially preferred.
The synthesis of zirconium chelates from 1,3-dicarbonyls and zirconium compounds as well as the use of the same is described, for example, in Houben-Weyl, xe2x80x9cMethods of Organic Chemistryxe2x80x9d [in German], 4th Edition, Vol. VI/2, 1963, pages 53-55 and 58 to 61, and also in A. E. Martell and M. Calvin, xe2x80x9cThe Chemistry of Metal Chelate Compoundsxe2x80x9d [in German](1958).
The inventors of the present invention have made the surprising discovery that catalytically active zirconium complexes can be synthesized in situ by addition to the reagents of inexpensive zirconium compounds such as tetrabutoxyzirconium and the 1,3-diketones described hereinabove, especially acetylacetonate.
Zirconium acetylacetonate is the most preferred catalytic compound for use in conjunction with the present invention.
A single type of zirconium catalyst can be used in the inventive process or alternatively mixture of different types catalysts may be used. The proportions of catalysts used for transesterification range from 0.01 to 10 mo1%, preferably from 0.05 to 10 mo1%, relative to one mole of 1,n-diols.
The (meth)acrylic acid esters of formula (II) are used particularly advantageously in proportions ranging from 2 to 20 mole, especially from 3 to 12 mole, per 1 mole of 1,n-diol.
Polymerization inhibitors may be used during transesterification to prevent undesired polymerization of the (meth)acrylates. These compounds, for instance, hydroquinones, hydroquinone ethers such as hydroquinone monomethyl ether, di-tert-butylpyrocatechol, phenothiazine, p-phenylenediamine, Methylene Blue or sterically hindered phenols are widely known among those skilled in the art. These compounds can be used individually or as mixtures and are generally commercially available. The proportion of inhibitors individually or as a mixture can generally range from 0.01 to 0.5% (w/w) relative to the weight of the entire reaction mixture.
Oxygen may also be used as an inhibitor. For instance, air containing oxygen may be introduced in a proportion such that the content in the gas phase above the reaction mixture remains below the explosion limit. Proportions ranging from 0.1 to 1 liter per hour per mole of 1,n-diol are especially preferred for this purpose.
The transesterification can take place at normal pressure, negative pressure or positive pressure. The reaction temperatures can be chosen within a wide range and generally depend on the pressure used. Advantageous temperatures range, for example, from 30 to 150xc2x0 C., especially from 50 to 130xc2x0 C. and most preferably of all from 70 to 120xc2x0 C.
Transesterification can be performed either continuously or batchwise. The inventive process can be performed in bulk, that is without using a further solvent. However, if desired, an inert solvent may be used. Examples of inert solvents include benzene, toluene, n-hexane, cyclohexane and methyl isobutyl ketone (MIBK), and methyl ethyl ketone (MEK), among others.
In a particularly expedient version of the inventive transesterification, all components, such as the 1,n-diol, the (meth)acrylic acid ester and the catalyst are mixed, after which this reaction mixture is heated to boiling. At this stage any water contained in the alcohol is first separated azeotropically with the starting (meth)acrylic acid ester. Thereafter the liberated C1 to C4 alcohol is removed from the reaction mixture by distillation, if possible as an azeotrope with that (meth)acrylic acid ester.
Reaction times depend on the chosen parameters, for example on pressure and temperature, on the reagents such as (meth)acrylic acid ester and 1,n-diol, and on the catalyst. In general, however, they range from 1 to 12 hours, preferably from 3 to 8 hours. The appended examples provide further information relating to reaction times for the person skilled in the art.
According to the present invention, the zirconium catalyst is separated from the mixture by addition of phosphoric acid after transesterification. The phosphoric acid can be mixed into the reaction mixture in pure form or as a solution. Addition of the phosphoric acid as an aqueous solution is particularly advantageous. The concentration of the aqueous solution can range, for example, from 0.5 to 90 wt %. The concentration also depends in particular on the 1,n-diol compound used, as can be inferred from the examples. The proportion of phosphoric acid used depends both on the zirconium compound used and on the 1,n-diol used. In general, it ranges from 0.5 to 2 moles per mole of zirconium compound to be precipitated.
A precipitate forms due to the addition of phosphoric acid. This precipitate can be separated from the reaction mixture by any method known to the person skilled in the art. Such methods include centrifuging, decanting, distillation and filtration among others. Filtration is especially preferred by virtue of its convenience and economy.
After the precipitate has been separated, the di(meth)acrylates obtained by the inventive transesterification generally exhibit, without purification by distillation, a zirconium content of less than about 1.5 ppm, preferably less than 0.7 ppm and most preferably of all less than 0.1 ppm, measured as the metal.
Depending on application, the di(meth)acrylic acid esters obtained in this way can be used without further purification. Low concentrations of residual 1,n-diol generally do not interfere with the subsequent polymerization reactions, and so conversions are substantially higher than 90% are usually achieved. The di(meth)acrylic acid esters are frequently used together with the acrylic starting substances, and so residues of these reagents do not have to be separated.
In this connection the obtained reaction mixtures are clear or substantially nonturbid after the catalyst has been precipitated by the phosphoric acid.
For special uses, however, the products obtained by the present process can also be purified by any method widely known among those skilled in the art.